This invention relates to telephone switching systems and more particularly to program controlled time division switching systems which operate on customer's premises or under other conditions where high reliability and self-maintenance are important.
In the years since the issuance of the Gebhardt et al U.S. Pat. No. 3,225,144, issued Dec. 21, 1965, a great revolution has taken place in the technology of telephone switching. The Gebhardt patent disclosed an early attempt at extending to the private branch exchange market the capability of time division switching technology. The commercial version of the system described in the Gebhardt patent is widely known as the ESS No. 101 and was manufactured by the Western Electric Company. The control of the establishment of telephone connections in the time division network of that system was effected by means of a stored program control unit which was most advantageously located in a central office. The network switches themselves were located in cabinets at the customers'premises and three or more switch units at diverse locations constituted the network of that system. The main controller at the central office had at its disposal a main memory unit which was divided, in accordance with the technology of the time, into changeable and semi-permanent memories. The changeable memory, termed a "call store", employed ferrite sheet storage and the semi-permanent memory, termed a program store, employed twistor technology. Both memory units were extremely reliable but, by present day standards, were somewhat bulky and characterized by slow access times.
As the main controller executed the call processing program stored in the program store, access was had to the call store and, in particular, to certain registers thereof which stored changeable progress mark words. The bits of a progress mark word either directly spelled out the address of, or were a pointer to the address of, the next-to-be-executed sequence of call processing programs that would be appropriate to the next phase or state of the particular type of call then being handled. Thus the progress mark could be interpreted either to identify how far a given call had "progressed" in its establishment and also, in a second sense, to furnish the addresses in the program store memory of the instructions necessary to further process the call.
More recently, time division switching systems have been built to serve customers whose installations require fewer telephones than the minimum size that could economically be handled by the rather large No. 101 ESS machine. A moderate size time division PBX switching system is disclosed for example, in D. G. Medill-J. F. O'Neill U.S. Pat. No. 3,789,152 issued Jan. 29, 1974. In that system, certain economies were achieved by employing a wired-logic rather than a stored-program controlled switching system. Both the main controller and the network were compact enough to reside on the customer's premises. In addition, each port appearing in the time division network had its own time slot storing shift register so that the network had an independent record of existing connections. The main controller of the Medill-0'Neill system from time to time issued a command to the network to identify the time slot number actually assigned to a given port in the network and this information was used by the central control to audit network operation.
More recently still, a time division switching system has been built which incorporates certain of the features of both the aforementioned Gebhardt et al and Medill-O'Neill patents. That system, certain aspects of which are the subject matter of the copending application of J. F. O'Neill Ser. No. 521,650 filed Nov. 7, 1974, now Pat. 3,916,118 issued Oct. 28, 1975 employs a stored program controlled processor, a fast access, semiconductor memory in which the programs as well as the progress marks that define the status of calls being handled are stored, and a time division switching network in which the individual ports have time-slot-defining shift registers. The semiconductor memory is deployed among one or more printed circuit boards removably mounted in the card carrier of a relay rack cabinet. The physical size of this switching system is sufficiently small so as to be capable of being unobtrusively located on the customer's premises. The system, in one of its embodiments, is capable of serving 512 network ports which may be apportioned among lines, trunks and service circuits in any desired combination.
While the art of maintenance processing is well developed, as exemplified by the teachings in the aforementioned Gebhardt et al and Medill-O'Neill patents, the problem of assuring reliable operation of a program-controlled switching system that is located on a customer's premises is somewhat more specialized than the maintenance arrangements appropriate to large, program-controlled central office switching installations such as the well-known No. 1 ESS described in the September 1964 Bell System Technical Journal. In small systems sufficient main processor sophistication must be provided to handle normal call processing requirements, including the specialized service features desired by some customers. The processor must also have sufficient capability to perform routine maintenance testing and verification. Certain conditions that are of likely, but infrequent, occurrence may, however, befall the switching system. To provide for their complete analysis and remediation would unduly complicate the processor or its programs. Nevertheless the system must be allowed to operate, on some basis, after such a condition has occurred. For example, when service personnel remove a semiconductor memory board from the card carrier the information content of the board is destroyed. When the card is reinstalled the semiconductor memory elements will be in random states and will represent meaningless information. It is envisioned that this may well take place while the system is in actual operation processing telephone calls. A stored program-controlled telephone switching system, unlike a general-purpose digital computer, is a real-time system and some means must be found to maintain system operation under these circumstances. It is important, moreover, to realize that incident to system recovery the processor may be quite severely taxed since it must perform restoration functions as well as handle real-time calls. The problem of preventing a processor from running out of "real time" as its occupancy is increased in the face of traffic conditions is dealt with in Eckhart-Hoover U.S. Pat. 3,623,007 issued Nov. 23, 1971. In that system, the number of items of new work taken from certain classes of hoppers is reduced by a flexible percentage as system occupancy increases beyond a given threshold. While such a system is appropriate for large and sophisticated stored program controlled systems it may not be efficient to incorporate that technique in smaller stored program controlled systems.